1. RF and feedback for bunch shortening
CARE HHH-APD WS
CERN, Nov 2004
J. Tückmantel, CERN
Abstr.: The motivation for, possibilities and limitations of bunch shortening
by an additional superconducting RF system at 1.2 GHz supplying
43 MV in LHC are examined. Possible technical realizations are examined
and critical issues shown

2. Contents Why shorter bunches, a heuristic sketch
Bunches in Buckets: which RF system
System parameters
Hardware considerations
Summary: what is ready to use, needs engineering or strong R&D effort
J. Tückmantel, 8 Nov 04

3. bunch shortening : 1/2 w. resp.
Desired:
bunch shortening : 1/2 w. resp.
2.5 eVs, 16 MV 400 MHz
-> 1.08 ns
J. Tückmantel, 8 Nov 04

4. Invest money elsewhere:
• Keep ‘good’ intra beam scattering lifetime:
-> blow up bunches to 2.5 eVs -> 1.08 ns (‘party-line’)
• Beam captured at 1 eVs (confident estimate, wait for 2007)
• If we understand LHC/SPS better, maybe eVs ( wait 200.)
1.0 eVs -> ns = 60% of 1.08 ns
0.7 eVs -> ns = 50% of 1.08 ns
-> goal reached ( but at 0.7 eVs)
Invest money elsewhere:
To be studied
Is beam still stable ???
Intra beam scattering ???
J. Tückmantel, 8 Nov 04

5. How to shorten bunches ? Increase RF voltage slope
Increase voltage of (existing) 400 MHz (brute force method)
4th-power ‘law’ (*) : for 1/2 b.l cavities ->16 * 8 = 128 cavities
Impedance increase: factor 16
Cost factor 16; no space in ring (& power converters) anymore
(technology copy/paste)
• Add system at higher frequency
The wavelength has to be longer than bunch length
The cut-off tube diameter has to be ≥ 5 cm (standard LHC pipe)
(in RF section as planned pipe diameter is 10 cm !!!)
RF manipulations -> multiple of basic RF frequency: n*400 MHz
--> 1200 MHz ( -> voltage: 43 MV )
(*) short bunches
J. Tückmantel, 8 Nov 04

6. 2.5 eVs: 16 MV @ 400 MHz (standard for IBS)
J. Tückmantel, 8 Nov 04

7. Brute force: 250 MV @ 400 MHz, 2.5 eVs
J. Tückmantel, 8 Nov 04

8. (only 43 MV @ 1.2 GHz: bucket too small !!)
±2440 MeV
2.5 eVs: MHz GHz
(only GHz: bucket too small !!)
J. Tückmantel, 8 Nov 04

9. Reduced emittance: 1.75 eVs
??? IBS ???
J. Tückmantel, 8 Nov 04

10. Brute force: 120 MV @ 400 MHz, 1.75 eVs
J. Tückmantel, 8 Nov 04

11. Reduced emittance : 1.75 eVs, double RF system: tight
±2120 MeV
Reduced emittance : 1.75 eVs, double RF system: tight
J. Tückmantel, 8 Nov 04

12. How to get to 7 TeV/c
Bunches of 1.75 eVs (2.5) with 400 MHz system only too long ≈0.9 ns (1.1) to be captured by 1.2 GHz system (‘wave length’ ns)
-> Transfer to 1.2 GHz has to be done before reaching the flat-top
Before start acceleration, in parallel:
- Ramp 400 MHz from 8 MV (capture) to 16 MV
- Ramp 1.2 GHz form 0 MV (enforced) to 5 MV
(‘abuse’ as Higher Harmonic System)
• Accelerate to ≈4.5 TeV/c, bunches shorten for higher Eb
• In parallel to continued acceleration to 7 TeV/c:
- Ramp 1.2 GHz to 43 MV
- (smooth) blow up of bunches -> 1.75 (2.5) eVs
--> Movie
J. Tückmantel, 8 Nov 04

13. <- Bunch shortening Open parenthesis
Bunch lengthening
(for different
Machine condition +
crossing angle)
J. Tückmantel, 8 Nov 04

14. Bunch 'lengthening' (Piwinski):
Bunch 4 eVs: MHz, 400MHz (HH) -> 2.7 ns
Ramp to 7 TeV/c as usual, then transfer > 4 eVs
J. Tückmantel, 8 Nov 04

15. Bunch shortening ->
Close parenthesis
Bunch shortening ->
J. Tückmantel, 8 Nov 04

16. Which cavity to choose ?
• 2 independent RF systems, 1 per beam (as 400 MHz)
Superconducting:
- high gradient -> less additional impedance
- as for 400 MHz system: beam gaps & reactive beam loading compensation -> low R/Q, high V best
The 400 MHz sc. cavity was designed for high currents: HOM modes (TM011) ‘tuned’, good HOM propagation into beam tubes, HOM coupling scheme can be scaled
--> scale 1:3 (& adapt) 400 MHz cavity
J. Tückmantel, 8 Nov 04

17. Scaling of 400 MHz cavity • Assume 25 MV/m (*) -> 3.1 MV/cav ->
(14 ->) 16 cavities at 2.7 MV do the job
• R/Qfundam. (identical) = 45 W
• long. HOMS (400 MHz calc. and f-extrapolated 3:1)
(assume Qext,damp as measured for 400 MHz):
2350 MHz 1.5 W*270 -> 0.4 kW/cav
2700 MHz 9 W* > 2.4 kW/cav
3200 MHz 4.5 W*1000 -> 4.5 kW/cav
3300 MHz 2 W* > 0.8 kW/cav
-> 8 kW/cav (some more)
(problem: cell to cell coupling …)
(*) 400 MHz: 5.5 MV/m -> 2 MV/cav
J. Tückmantel, 8 Nov 04

18. Fundamental mode impedance:
Qext,max = , R/Q = 45W -> bare Z = 9 MW/cavity
(16*9 MW = 144 MW)
Fast RF vector feedback: delay in loop T=600 ns (tunnel layout)
gain limited to half of ginst= Qext/(2 fRF T)
-> ZFB= 4 (R/Q) fRF T = 130 kW / cav
(16*130 kW = 2.08 MW)
The 1-turn-delay F.B. may give another factor 3 … 5 (comb profile) : use coming experience of nominal LHC with 400 MHz
J. Tückmantel, 8 Nov 04

19. TESLA Technology at 1.3 GHz
25 MV/m reliably in ‘machine’ (more was done: LHC needs ‘absolute’ reliability !!)
Bulk Nb cavities, run at 2 K (not 4.5 K)
Q0 ≥ 1010
TESLA is pulsed: 5 (or 10) Hz with 1.4 ms RF-on time per pulse -> average LHe cons. limited
LHC in CW : ≈ 250W dynamic 2K
-> cryostat through-put !!
TESLA uses 9-cell cavities
-> LHC needs 1-cell cavities (fast RF feedback !!)
J. Tückmantel, 8 Nov 04

20. RF power : variable coupler Qext
To make cavity ‘invisible’ (e.g. injection),
lowest Qext -> Qext,min = 10,000
(else antenna ‘scratches’ beam)
Detune (far away) cavity, possib. without power ?
-> Impedance is still somewhere -> CB instabilities
-> At one instant cavity has to be brought to tune
• 400 MHz system Qext-stroke = factor 20
Qext,max = 200,000
J. Tückmantel, 8 Nov 04

21. Beam current options Full B.L. = 4*3.8 cm = 15.2 cm
At 1.2 GHz RF current reduced (cos2) by
0.78 with respect to point bunches
• (n)ominal: 0.56 A (DC)
(u)ltimate: 0.85 A (DC)
(s)uper: A (DC)
Keep cavity ‘invisible’ on tune (force V=0):
-> n: 43 kW; u: 100 kW; s: 275 kW per cavity
J. Tückmantel, 8 Nov 04

22. Necessary RF power (for V=2.7 MV)
current
Qext,opt
(half-detuning)
<P> (*)
(full control)
P (adapting FB) (+) Q=Qmax n
135000
300 kW
… -> 100 kW u
90000
450 kW
… -> 100 kW s
55000
750 kW
… ->100 kW
(*) Power spikes: about 50% more installed power !!
(+) OK in theory and ‘honest’ simulation program
(with 400 MHz system)
Will be very delicate to handle in reality
( … beam loss …)
-> No guarantee, has to be tried out in truly existing LHC
J. Tückmantel, 8 Nov 04

23. RF Power converters Info Erk Jensen (no exhaustive research) >
TESLA MBK's typically 150 kW CW
(1.3 GHz)
Raytheon's "Amplitron": 425 kW CW
(3 GHz) 76 % efficiency
(for ‘s’ also the 400 MHz needs an upgrade above 300 kW !!)
J. Tückmantel, 8 Nov 04

24. The Power coupler Problem: sc. cavities may reflect full power
(e.g. when fast vector feedback wants to lower field)
On coupler (and transmission lines) fields may double
i.e. local power may be 4 times nominal one
[ average power - thermal - OK, but arcing ]
400 MHz system: Pnom = 300 kW, peaks 1.2 MW:
Tests show it should work, but at limit, long processing
1.2 GHz: already for ‘nominal’ we need for full control
more than 300 kW at 1.2 GHz
J. Tückmantel, 8 Nov 04

25. LHC 400 MHz single cell cavity (Nb/Cu technology)
J. Tückmantel, 8 Nov 04

26. High field: bulk Nb, TESLA technology (1.3 GHz)
J. Tückmantel, 8 Nov 04

27. J. Tückmantel, 8 Nov 04

28. J. Tückmantel, 8 Nov 04

29. 400 MHz sc. cavity module with 4 cavities
Main coupler
HOM coupler
power outlet
Main RF inlet
(door knob)
accelerated beam
‘guest’ beam
400 MHz sc. cavity module with 4 cavities
J. Tückmantel, 8 Nov 04

30. LHC 400 MHz 4-cavity module (mirrored..)
J. Tückmantel, 8 Nov 04

31. Challange: HOM outlet lines, 'density' of wave-guides and couplers
Each cavity has 1 MC (>300 kW)
4 HOM couplers, 1 tuner, …
Challange: HOM outlet lines, 'density' of wave-guides and couplers
J. Tückmantel, 8 Nov 04

32. Challange: Cryo insulation (+ see above)
Each cavity has 1 MC (>300 kW)
4 HOM couplers, 1 tuner, …
Challange: Cryo insulation (+ see above)
J. Tückmantel, 8 Nov 04

33. Technology Summary: Ready: Cavity design: scale 400 MHz cavity
- HOM coupling scheme: scale (HOM power ?? )
1-cell sc. cavities at 1.2 GHz, bulk Nb :
TESLA technology 25 MV/m: reliable
Cryo supply at 2 K (TESLA, CEBAF, … LHC)
Tunnel length needed : m (16 cav parall.)
J. Tückmantel, 8 Nov 04

34. • No fundamental problems (but to be engineered/checked):
Cryostat with a multitude of ‘outlets’ (‘hedgehog’)
cryo-losses ?
mounting a ‘crowded’ RF supply system ?
Allocation of power converters, wave guide passage
into tunnel (radiation !!)
––––––––
Check beam-stability issues
-Check intra beam scattering issues (2.5 -> 1.75 eVs)
J. Tückmantel, 8 Nov 04

35. (possible show-stoppers):
• R&D challenges
(possible show-stoppers):
RF power converter kW CW at 1.2 GHz
(combining smaller ones ugly and expensive…)
Main coupler kW CW at 1.2 GHz
(superimposed: spikes of up to 4 times this value)
Diameter 1/3 of known 400 MHz/300 kW system:
3 * field-strength, 9*√3 ≈ power density
and finally ….
Run LHC with double RF system with fast vector FB
without losing coasts too often
(integrated luminosity counts !!)
J. Tückmantel, 8 Nov 04